The present invention relates to a molecular sieve comprising crystals or agglomerates whose average largest dimension is of the order of nanometers, and processes for its production.
Molecular sieves are microporous crystalline materials which find many uses in chemical processes. Zeolites, generally based on crystalline aluminosilicate, are a well known class of molecular sieve. For some purposes, the use of small zeolite crystals or agglomerates of crystals is desirable and the intrinsic quality of the zeolite generally improves as the crystal size is reduced. For commercial purposes it is desirable that the crystal or agglomerate size is substantially uniform and the production of small crystal molecular sieves should be accurate and reproducible with respect to the crystal size. It would also be useful for some purposes if the crystals or agglomerates were sufficiently small and uniform that the molecular sieve was capable of forming a colloidal suspension. The applicants have identified a new form of molecular sieve which solves these problems. The applicants have also identified a controllable way to produce such material.
The production of molecular sieves having small crystals has been described in a number of documents. For example, EP-A-173901 describes the production of ZSM-5 zeolite whose crystals are “below 0.3 μm” in size. However, the specific zeolites whose production is described are made up of “aggregates of crystallites ranging in size from about 0.1 to 0.3 μm”.
U.S. Pat. No. 4,205,053 describes a process for preparing zeolites such as ZSM-5 and ZSM-35. The smallest crystals produced have a size of 0.2 to 0.5 microns, or are lamellae or lamellar intergrowths of about 0.1 μm in size.
U.S. Pat. No. 3,781,226 and U.S. Pat. No. 3,926,782 describe the production of zeolites KL and ZSM-5. Although the size of crystals produced (0.005 to 0.1 μm) appears to be such that it would be expected that the crystals would form a stable colloidal suspension, the crystals form agglomerates having a size of 0.1 to 1 μm and these agglomerates do not form a stable colloidal suspension. A stable colloidal suspension is one in which the crystals or agglomerates do not visibly separate out of the suspension when the suspension is left for a prolonged period e.g. left standing for a month.
U.S. Pat. No. 4,526,879 describes the synthesis of a low sodium zeolite ZSM-5 from a mixture containing sources of an alkali metal oxide, an aluminum oxide, a silicon oxide, and a combination of amine, a halide and a mutual solvent. Although the crystals produced are stated to be 0.05 to about 20 microns in diameter it is believed that the crystals produced are, in fact agglomerates. No means of controlling the crystal size or uniformity is disclosed.